A machine tool, particularly, a bed of a turning center is manufactured by a casting forming method, and as illustrated in FIG. 1, a main shaft housing 20 in which a work piece is chucked, a turret 40 and a tool table 50 for processing the work piece by a tool, a transfer system or a transfer apparatus 30 for transferring the turret 40 and the tool table 50 for each axis (X, Y, Z) direction are assembled at an upper portion of a bed 10.
In a case of a general machining center, because the machining center has a symmetric structure, a control for thermal displacement of the structure according to a change in external temperature may be compensated in real time, but as illustrated in FIG. 2A, because the bed 10 of the turning center has an asymmetric structure in shape, and particularly, since positions of the transfer apparatus 30 and the main shaft housing 20 form an asymmetric structure, the real time compensation is difficult and static stiffness according to an action direction of maximum cutting force needs to be secured. Further, there is a rib in the bed 10, and a general shape of the rib of the bed is designed to be a rectangular box shape, and the rib is designed so that an interval of the rib is adjusted in accordance with a weight of an assembly at an upper portion of the bed.
As illustrated in FIG. 2B, because of the asymmetric structure of the turning center, thermal deformation according to a change in external temperature shows a shape in which a linear mode and a nonlinear mode are combined, and nonlinear deformation makes a control for thermal displacement of the turning center difficult. In addition, in a load condition considering a condition of the maximum cutting force, a static stiffness characteristic is also not good.
FIGS. 3A and 3B illustrate a shape of the bed of the turning center and a shape of the rib in the bed of the related art.
Because the shape of the bed 10 of the turning center of the related art is designed considering only the static stiffness in the condition of the maximum cutting force without considering an influence of the change in external temperature, as illustrated in FIG. 3A, the bed 10 of the turning center of the related art has an upper and lower two stage structure including an upper bed 11 and a lower bed 12, and as illustrated in FIG. 3B, the bed 10 has an equal interval shape in which a ratio of intervals of rib elements 11-1(12-1), 11-2(12-2), 11-3(12-3), and 11-4(12-4) in the bed 10 is 1:1:1:1.
However, in the bed structure of the turning center of the related art, a relative thermal displacement amount at end points of the work piece and the tool shows a considerably high value (for example, 20 to 30 μm) in accordance with a test condition, and a higher thermal deformation amount is shown in a high speed and high precision processing condition.
One of methods for reducing a temperature gradient of a machine tool bed and the thermal displacement due to the temperature gradient is a method of improving a material of the machine tool bed such as a method of using a casting material having an excellent thermal characteristic. However, there is a problem in that the above method causes a heavy burden in terms of cost, and the thermal displacement due to occurrence of a local temperature gradient according to a change in thermal environment may not be basically and greatly reduced by only the above countermeasure.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.